The use of adaptive or smart antennas is considered as one of the key features for increasing coverage and capacity of a wireless or cellular system. When beamforming is applied in the base station several narrow beams, compared to the sector/cell beam, may be created to maintain coverage in the cell. FIG. 1A illustrates a sector cell antenna beam. Although a sector antenna is useful to communicate broadcast and/or control information to all mobiles in the sector cell, an adaptive antenna may be used to transmit and receive in narrow beams covering just a part of the sector cell. FIG. 1B shows an example of a narrow antenna beam. FIG. 2 illustrates an example of a cellular network with a base station transmitting a sector beam, a base station transmitting one of the possible beams in a multi-beam system, and a base station transmitting a steerable beam. Some benefits of adaptive antennas are shown in FIG. 3, where a narrow beam of the adaptive antenna may be directed to an intended mobile and therefore spreads less interference in the download direction. The narrow beam also suppresses spatial interference from adjacent cell interferers in the uplink direction. Both factors increase the signal-to-interference gain, and therefore increase the overall system performance.
HSDPA (High Speed Data Packet Access) is another important feature that enables improvements in capacity and end-user perception by means of efficient sharing of common resources in the cell among many users, rapid adaptation of the transmission parameters to the instantaneous radio channel conditions, increased peak bit rates and reduced delays.
Fast scheduling is the mechanism selecting which users to transmit to in a given transmission time interval (TTI). The scheduler is a key element in the design of a HSDPA system as it controls the allocation of the shared resources among the users and to a great extent determines the overall behavior of the system. In fact, the scheduler decides which users to serve and, in close cooperation with the link adaptation mechanism, which modulation, power and how many codes should be used for each user. This produces the actual end-users bit rate and system capacity.
In order to improve the system capacity in terms of total cell throughput, the scheduling algorithm normally bases its decision primarily on the channel conditions experienced by the user equipment (UE); however, it can be designed in a flexible way so as to consider other aspects such as the fairness from a time resource perspective or the average bit rate.
In a WCDMA HSDPA system, a channel quality indication (CQI) is reported by the UE and used for scheduling and link adaptation. Since the CQI is strictly connected to the quality of a common pilot signal, it is strongly affected by the interference levels.
The fast scheduler, normally located in the radio base station (RBS), targets which users to serve in each TTI and distributes the available channelization codes and power resources among the selected users. In case code multiplexing is applied, more than one user can be served in the same TTI by using distinct parts of the set of channelization codes allocated for the HSDPA related channels. Several scheduling algorithms that can be used to enforce specific strategies in terms of trade-off between system capacity and user fairness are available in the literature. The most common algorithms are described below.
The round robin (RR) algorithm allocates radio resources to the users on a sequential basis and it does not base its decision on the instantaneous radio channel conditions experienced by the connection. The system performance is not maximized even though a certain degree of fairness is obtained in terms of access to the radio resources.
The proportional fairness (PF) scheduler better exploits the channel conditions and ensures that all users receive a guaranteed minimum throughput, providing fairness among users together with system performance improvement. The scheduler transmits information to some users based on CQI information, delay and other measurements.
The maximum C/I algorithm bases the user selection solely on the CQI information reported by the UE.
The CQI report in WCDMA multi-beam antenna system is based on the quality of the Secondary Common Pilot Channel (S-CPICH).
However, existing solutions do not enable the system to benefit from the capacity/coverage gains promised by narrow beam/adaptive antenna techniques.